Modular synths are organized around a fundamental principle: individual modules provide a single/finite number of functions that vary depending on how they're patched. From this, all finite functions can combine in seemingly infinite ways, providing an open ended music making tool. This system is not unlike organs in the body, which are fixed in their function, but can work within a limitless number of species of animals. Modules are usually connected to a common power supply, affixed inside a case, and connected to one another via patch cables. These patch cables decide how the instrument is constructed, with new patches essentially creating new "creatures." This means the architecture of ones synthesizer can always evolve. New modules can be added as new functionality is desired, and your modular synthesizer can grow and change with ones needs. Fortunately, the power supply specifications are all standardized, so any eurorack module can work with any eurorack power supply. The signal levels are also standardized, allowing any output to be patched into any input. With this openness, it is also possible to create patches that do not function, this is where research and signal flow theory become essential, especially with so many diverse functions available in the Eurorack modular format. You can find the best selection of modular synth systems, modules, and DIY kits in the Eurorack format at Perfect Circuit Audio.